Great question. I'm rally interested in the answer here, too. There has been some chatter that if you're going to place both a 6th and 7th channel in the rear, then the M2s are the way to go, because there will be some directionality, but the sound field will be spread out due to the distance between the #s 6 & 7, firing simultaneously in mono.
I think that they also may even out your listening room if you do decide to get into 8 channel audio, thus eliminating a front and back of the soundstage. (You'll have direct firing speakers in front and rear, and diffuse, dipoles in the middle).
BTW - your room sounds awesome for HT. Wish I was lucky and had a great big room with lots of extra space on back walls for this.
